The Brunswick magnetic anomaly: Geophysical signature and geologic source

Abstract

We used integrated analyses and forward and inverse modeling of potential field data to interpret the enigmatic Brunswick magnetic anomaly (BMA), a prominent negative magnetic lineation that is developed within the ocean-continent transition of the Southeast Georgia Embayment. From magnetic profiles extracted from gridded data, residual magnetic maps, and simultaneous 2.5-dimensional forward and 3-dimensional Euler inverse modeling of the gravity and magnetic fields, we suggest that the source of the BMA is a series of semicontinuous to discrete, late-stage rift-related mafic intrusions of Mesozoic age. This interpretation is supported by the observation that the BMA is independent of the East Coast magnetic anomaly, implying a predrift source, and that the amplitude and frequency of the anomaly change nearshore across a fracture zone that divides the offshore BMA, where continental breakup occurred, from the onshore BMA. Modeling demonstrates for the first time that a Mesozoic rift-related mafic body can explain the anomaly onshore. The lack of a BMA equivalent on the West African margin may indicate that Atlantic rifting began with a single lithospheric dislocation. INTRODUCTION The Brunswick magnetic anomaly (BMA), located in southern Georgia and the adjacent offshore Southeast Georgia Embayment, forms a prominent geophysical feature along the southeastern North American margin. The origin of the negative BMA is enigmatic because the anomaly has no equivalent along the conjugate West African margin, and because of the lack of a consistent magnetic model that explains both its onshore and offshore segments. This paper explains the origin of the BMA with new potential field analyses that support a unified interpretation of the onshore and offshore portions of the BMA, highlighting the importance of mafic magmatism in the asymmetrical rifting of Pangea just prior to opening of the Atlantic. In previous work (e.g., Hutchinson et al., 1983, 1990; McBride and Nelson, 1988; Austin et al., 1990; Holbrook et al., 1994; Lizarralde et al., 1994; Parker, 2014), the BMA was divided into onshore and offshore segments because interpretations were based on seismic surveys that did not traverse the shoreline, and because of the anomaly’s apparent association with compressional structures onshore and riftrelated structures offshore. Onshore, the BMA developed near the boundary between the peri-Gondwanan Charleston and Gondwanan Suwannee terranes, which has been interpreted as a Paleozoic suture (Chowns and Williams, 1983; Dallmeyer et al., 1987; Boote et al., 2018). Offshore, the BMA appears inboard and adjacent to the hinge zone within the ocean to continent transition, and it parallels the East Coast magnetic anomaly (ECMA), which is continuous from offshore Georgia to Nova Scotia. The ECMA is associated with packages of volcanic seaward-dipping seismic reflectors that are often included as part of the Central Atlantic magmatic province (CAMP), a large igneous province emplaced within the Atlantic rifted margin (Davis et al., 2018; Labails et al., 2010; Austin et al., 1990). The ECMA marks the landward limit of Atlantic oceanic crust and the locus of the onset of seafloor spreading (Hutchinson et al., 1983; Bird et al., 2007), which have led some authors to view the BMA and ECMA as a low-high paired magnetic anomaly (McBride and Nelson, 1988; Austin et al., 1990). Along U.S. Geological Survey (USGS) seismic reflection line 32 (USGS-32, Fig. 1), Hutchinson et al. (1983) used gravity and magnetic data and northwest-dipping reflections to model the source of the BMA as the Brunswick graben, a Mesozoic (?) rift basin. However, the graben is not imaged in the nearby BA-6 seismic profile of Austin et al. (1990), who attributed the dipping reflectors to scattering artifacts from out-of-plane features. Instead, Austin et al. (1990) attributed the BMA to an edge effect from remanently magnetized oceanic crust seaward of the hinge zone. Holbrook et al. (1994), using wide-angle-reflection data along BA-6, explained the BMA as an edge effect from highly magnetized intruded and underplated transitional crust against relatively low-susceptibility rifted continental crust. The BMA appears locally near zones of southsoutheast–dipping intracrustal (to ~30 km depth) reflectivity imaged from onshore Consortium for Continental Reflection Profiling (COCORP) seismic data and offshore Brunswick anomaly (BA) seismic data, interpreted to be a crustal-scale imbricate structure (McBride and Nelson, 1988; Austin et al., 1990). McBride and Nelson (1988) modeled the source of the BMA as a seawarddipping slab of high magnetic susceptibility, interpreted to be associated with the Alleghanian suture between Laurentia and Gondwana. Parker (2014) modeled the source of the BMA as longlived relatively weak reverse-polarity remanent magnetization of lower-crustal rocks resulting from transpressional motion during the initial stage of Alleghanian collision. Recently, Boote et al. (2018) suggested a connection between the BMA and dipping reflectivity, which they interpreted to mark a preserved subduction zone of Neoproterozoic age on the basis of overlapping Gondwanan Suwannee Basin Paleozoic strata. METHODS Gravity data for this study are from the USGS U.S. Gravity Database, and magnetic data are from the North American Magnetic Map (see the GSA Data Repository1). Processed COCORP and USGS Seisdata-8 seismic reflection data, as well as USGS-32 and BA seismic data, are from published literature (Nelson et al., 1985; McBride and Nelson, 1988; Behrendt, 1986; Hutchinson Manuscript received 8 August 2018 Revised manuscript received 27 January 2019 Manuscript accepted 29 January 2019 https://doi.org/10.1130/G45462.1 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online XX Month 2019 1GSA Data Repository item 2019122, gravity and magnetics data, rock properties, and modeling methods, is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Duff, P.D., and Kellogg, J.N., 2019, The Brunswick magnetic anomaly: Geophysical signature and geologic source: Geology, v. 47, p. 1–4, https:// doi .org /10 .1130 /G45462.1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4655136/g45462.pdf by Patrick Duff on 01 March 2019 2 www.gsapubs.org | Volume 47 | Number 4 | GEOLOGY | Geological Society of America et al., 1983; Austin et al., 1990; Holbrook et al., 1994; Lizarralde et al., 1994). Seismic refraction data are from the EarthScope SUGAR (Suwanee Suture and GA Rift basin experiment) project (Marzen et al., 2016). Magnetic susceptibilities and densities are from measurements on sample lithologies from the region, and from seismic velocities (see the Data Repository). The interpretation of the BMA represented here was derived by filtered total magnetic intensity grids, and forward and inverse modeling of Bouguer gravity and total field magnetic data. High-band-pass filtering of the total magnetic intensity data isolated wavelengths less than 50 km. Model polygon geometries for forward modeling were constrained along a 2.5-dimensional (2.5-D) model profile using three boreholes to basement, Seisdata-8 seismic reflection data, and a preliminary SUGAR velocity model (Fig. 1). The location of the forward model profile was chosen to traverse a local magnetic minimum along the BMA and the adjacent magnetic high of the Tifton anomaly (Fig. 1; Chowns and Williams, 1983). Analysis and modeling of gravity and magnetic data were performed using Geosoft software. GM-SYS forward modeling software (https:// www .geosoft .com /products /gm -sys) computes the gravitational and magnetic anomalies for a given polygon geometry, density, and magnetic susceptibility. Inverse modeling of total field magnetic data was performed by 3-dimensionally located Euler deconvolution (see the Data Repository). RESULTS The filtered magnetic intensity grid (Fig. 2) reveals the BMA to be a paired low-high magnetic anomaly inboard and independent of the ECMA, and it shows that the amplitude of the magnetic low weakens from offshore to onshore. An along-strike profile of the offshore and onshore extent of the BMA from total field magnetic data reveals an anomaly amplitude in the hundreds of nanoteslas, suggesting a basement source (Fig. 1B). Offshore, the BMA displays low-frequency and high-amplitude signatures, which give way to high-frequency and lowamplitude signatures onshore (Fig. 1B). The division between the two segments the BMA coincides with the Blake Spur fracture zone (BSFZ), a transfer zone between Mesozoic rift segments active during late-stage rifting to incipient seafloor spreading along the margin (Mutter and Detrick, 1984; Minshull et al., 1991). Forward modeling of the BMA in this study tested for the first time a rift-related mafic intrusive interpretation onshore, similar to previous offshore models. The forward model (Fig. 3) produces a good fit between observed and calculated values for both gravity and magnetic anomalies with a single source geometry (Fig. 3). The mafic pluton source is 50 km wide and 6 km thick, with an upper surface at 1 km depth and a basal contact at 7 km depth. A magnetic susceptibility of 0.012 cm gram seconds (cgs) and a density of 3.0 g/cm3 were used for the intrusive body, and the source was modeled in 2.5-D because of the elliptical shape of the magnetic anomaly (Tifton anomaly, Fig. 1). The susceptibility value is within the range for diabase/gabbro compositions, a rock type known from well data to be present in the subsurface of South Georgia (Chowns and Williams, 1983). Davis et al. (2018) demonstrated that varying-polarity basalt layers, such as those measured in CAMP basalts at Clubhouse Crossroads (Fig. 1) by Phillips (1983), can cause the remanent anomalies of the layers to cancel out, preventing high-amplitu